Season the beef generously with salt and pepper.
Heat the canola oil in a large Dutch oven or heavy pot over high heat until it begins to shimmer.
Add the beef in batches, browning each side well, about 5 minutes per side, then set aside.
Add carrots, onion, celery, and garlic to the hot pot and cook, stirring often and scraping the pan to loosen browned bits, until the vegetables begin to brown, about 6 minutes.
Transfer the cooked vegetables to a heatproof bowl and set aside.
Pour in the chicken stock, bring to a boil, and scrape the bottom of the pot to lift any flavorful residue.
Cut the browned beef into chunks and return to the pot along with any reserved bones and the herb sachet.
Lower the heat to a gentle simmer, cover partially, and cook until the beef is tender, between 1 and 2 hours, skimming off foam as needed.
Remove and discard the bones and herb sachet from the pot.
Add the pearl barley, reserved vegetables, and fish sauce if using; continue to simmer until barley and vegetables are fully tender, about 30 minutes.
Adjust the seasoning with salt and pepper to taste.
If the soup is too thick, add water gradually to reach your preferred consistency.
Serve piping hot garnished with freshly minced parsley.
